include signal.h

linux included it indirectly somehow, but relying on that is wrong and
breaks the freebsd build.

src/libs/utils/process_stub_unix.c

@@ -35,6 +35,7 @@
 #include <sys/ptrace.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <signal.h>
 #include <string.h>
 #include <stdlib.h>
 #include <stdio.h>